A Study on Ferroresonance Mitigation Techniques for Power Transformer S. I. Kim, B. C. Sung, S. N. Kim, Y. C. Choi, H. J. Kim Abstract--This paper presents a comprehensive study on the ferroresonance mitigation techniques for a power transformer by performing four feasible solutions as follows: (a) an increase of capacity of shunt capacitance at the transformer primary side (b) a change of the transformer saturation characteristics with the low flux density (c) an insertion of the capacitor bank at the transformer secondary side (d) an installation of the resistive load bank at the transformer secondary side. In order to verify the methods mentioned above, the EMTP-RV (Electromagnetic Transients Program – Restructured Version) software is used to model the each study cases. This paper also introduces some reviews on isolating a power transformer using a disconnector, investigating the operation of surge arresters against ferroresonance overvoltage, and examining the variation of the residual flux of the iron core in a power transformer. Keywords: ferroresonance, mitigation technique, power trans- former, EMTP-RV. I. INTRODUCTION ERRORESONANCE is a non-linear resonance phenomenon which can be caused in a low loss electric circuit containing non-linear inductance, capacitance, and a voltage source. Non-linear inductance consists of power transformers, inductive voltage transformers and so on. Capacitance is made of cables, long transmission lines, power transformers, and grading capacitors in circuit breakers. Contrary to inductive voltage transformers, the failure of power transformers due to ferroresonance overvoltages has not yet reported [1]. The sustained overvoltage under ferroresonance, however, can accelerate the deterioration of the insulation materials in power transformers and result in the failure of surge arresters. In order to avoid unexpected equipment damages, the effective and practical techniques should be studied. An extensive literature on ferroresonance provides a number of countermeasures against ferroresonance in power transformers. Among these measures, four feasible solutions are examined in order to prevent the actual ferroresonance for the 25 MVA 380 / 13.8 kV YNyn0 auxiliary power transformer which is presented in Chapter II. This paper also introduces some reviews on isolating a power transformer from the grid during ferroresonance using a disconnector between them, investigating the effectiveness of the surge S. I. Kim, B. C. Sung, S. N. Kim, Y. C. Choi, and H. J. Kim are with Hyundai Heavy Industries Co., Ltd., Korea (e-mail: [email protected]). Paper submitted to the International Conference on Power Systems Transients (IPST2015) in Cavtat, Croatia June 15-18, 2015 arresters for the actual ferroresonance, and examining the variation of the residual flux of the iron core in a power transformer. II. FERRORESONANCE OF POWER TRANSFORMER WITH CIRCUIT BREAKER GRADING CAPACITORS A ferroresonance phenomenon was experienced during a normal operation of the auxiliary power transformer bay in the 380 kV substation. The single line diagram of the 380 kV circuit to the 380 / 13.8 kV auxiliary power transformer is shown in Fig. 1 where the power transformer is connected to GIS (Gas Insulated Switchgear) via XLPE (Cross-Linked Polyethylene) cables. Aux. Transformer 25 MVA 380 / 13.8 kV YNyn0 380 kV BUS-1 380 kV CB 380 kV BUS-2 87T 120 m 380 kV XLPE 630 mm 2 Cable 13.8kV CB TR Differential Relay 15 m 13.8 kV XLPE 185 mm 2 Cable 360 kV Surge Arrester 360 kV Surge Arrester To 260 MVA Generator • Note: All 380 kV circuit breakers include grading capacitors. The ferroresonance phenomenon was disappeared after opening the line disconnector (DS*) between the transformer and the CBs. 380 kV CB 380 kV CB 10 ohm To 13.8kV Power System DS DS DS DS DS DS DS DS* Fig. 1. Single line diagram of 380 kV circuit to 380 / 13.8 kV transformer. The ferroresonance phenomenon was caused by an abnormal operation of the differential relay for the power transformer protection during the normal operation. Due to the sudden operation of the circuit breakers, the above 380 kV circuit is changed to a series non-linear resonance circuit with magnetizing inductance (L non ) of the power transformer, branch capacitance (C b ) of the cable and the power transformer, series capacitance (C s ) of the two parallel grading capacitors in circuit breakers, and resistance (R) of the load. Despite opening the circuit breakers, the series non-linear resonance circuit was energized through the grading capacitors, which operates as a source in the circuit. C b C s L non Circuit Breaker Source (V s ) R Fig. 2. Series non-linear resonance circuit. F
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A Study on Ferroresonance Mitigation Techniques for Power Transformer
S. I. Kim, B. C. Sung, S. N. Kim, Y. C. Choi, H. J. Kim
Abstract--This paper presents a comprehensive study on the
ferroresonance mitigation techniques for a power transformer by
performing four feasible solutions as follows: (a) an increase of
capacity of shunt capacitance at the transformer primary side (b)
a change of the transformer saturation characteristics with the
low flux density (c) an insertion of the capacitor bank at the
transformer secondary side (d) an installation of the resistive load
bank at the transformer secondary side. In order to verify the
methods mentioned above, the EMTP-RV (Electromagnetic
Transients Program – Restructured Version) software is used to
model the each study cases. This paper also introduces some
reviews on isolating a power transformer using a disconnector,
investigating the operation of surge arresters against
ferroresonance overvoltage, and examining the variation of the
residual flux of the iron core in a power transformer.
Keywords: ferroresonance, mitigation technique, power trans-
former, EMTP-RV.
I. INTRODUCTION
ERRORESONANCE is a non-linear resonance
phenomenon which can be caused in a low loss electric
circuit containing non-linear inductance, capacitance, and a
voltage source. Non-linear inductance consists of power
transformers, inductive voltage transformers and so on.
Capacitance is made of cables, long transmission lines, power
transformers, and grading capacitors in circuit breakers.
Contrary to inductive voltage transformers, the failure of
power transformers due to ferroresonance overvoltages has not
yet reported [1]. The sustained overvoltage under
ferroresonance, however, can accelerate the deterioration of
the insulation materials in power transformers and result in the
failure of surge arresters. In order to avoid unexpected
equipment damages, the effective and practical techniques
should be studied.
An extensive literature on ferroresonance provides a
number of countermeasures against ferroresonance in power
transformers. Among these measures, four feasible solutions
are examined in order to prevent the actual ferroresonance for
the 25 MVA 380 / 13.8 kV YNyn0 auxiliary power
transformer which is presented in Chapter II. This paper also
introduces some reviews on isolating a power transformer
from the grid during ferroresonance using a disconnector
between them, investigating the effectiveness of the surge
S. I. Kim, B. C. Sung, S. N. Kim, Y. C. Choi, and H. J. Kim are with Hyundai